Seven people were killed and 51 injured in the pile-up involving 34 vehicles near Taunton on Friday night. The motorway was fully reopened on Sunday night after forensic work on the northbound carriageway was complete and a 60m section that had been damaged by fire was repaired.

An Avon and Somerset police spokeswoman said on Monday that 11 people hurt in the incident remained in hospital in a stable condition.

The West Somerset coroner, Michael Rose, said he planned to open inquests into the deaths of the seven victims of the crash on Thursday in Taunton. "I have requested three forensic pathologists to carry out postmortems today on the seven victims of the tragic incident on the M5," he said.

There was no bonfire, but police believe smoke from the fireworks could have drifted on to the motorway. Bangham said there was fog in the area but witnesses told police there was "very significant smoke across the carriageway. This caused a bank similar to a fog bank, which was very distracting and very difficult to drive through," he said. Asked if the display was too close to the M5, he said: "The display was certainly very close."

The rugby club has said previously that its display was over by 8.15pm, 10 minutes before the time of the crash.

A statement released by the club said: "Taunton rugby club would like to take this opportunity to extend our thoughts and prayers to the families and friends of those who have been affected by the tragic incident on the M5 motorway on Friday 4 November. As a family-based community club, Taunton RFC held a fireworks display on the evening of the 4 November. Taunton RFC is working closely with Avon and Somerset police to assist with their investigations."

The company that provided and lit the fireworks is reported to have insisted that no more smoke than normal was created. It was not available for comment on Monday. Colin Bentley, from the club's promotions and marketing team, told the Somerset County Gazette on Sunday: "Everybody at the club is incredibly upset at the terrible loss of life – no one could fail to be moved by it. My heart, soul and prayers go out to those affected. The club is supporting the police in their inquires. I don't believe the club has done anything improper. Checks and risk assessments were done."

Taunton Deane council said it was working closely with police. It confirmed that fireworks displays were not licensable activities under the Licensing Act. A spokeswoman said: "The Licensing Act covers a wide range of activities from live music to the sale of alcohol but does not encompass fireworks displays."

The council leader, John Williams, said: "We are working closely with the police and other agencies. It would be inappropriate to comment further at this stage. On behalf of the council, I would like to extend my deepest sympathy to the families of the bereaved and the families and friends of all those affected by these terrible events."

Somerset county council lowered its flag outside its county hall headquarters as a mark of respect to the victims. Ken Maddock, the council leader, said: "We have enormous sympathy for all those involved in this terrible accident. Our thoughts are with everyone affected by this tragic event. I would also like to give my overwhelming thanks to the emergency services for their outstanding efforts. We are all united in grief."

Tom Smith, secretary of the British Pyrotechnists Association, said it was "very rare" for smoke to be a major issue. He doubted that a relatively small fireworks display could have created the bank of smoke that police had described.

Smith accepted smoke could have been a contributing factor, coupled with fog and mist rising from the river Tone that flows under the M5 close to where the pile-up happened. But he also pointed out that video he had seen of the aftermath seemed to suggest there was wind about, which would have led any smoke to disperse swiftly. "Safety is paramount. It's very rare for smoke to be a major issue," he said.

The transport minister, Mike Penning, said it was possible that some witnesses who reported smoke were confusing what had happened after the incident with the conditions before it. He said: "Once these lorries caught fire, diesel produces a very black acrid smoke, the tyres were alight, the plastics were alight. That mixing with mist or fog would create something that people might have thought was there before but was not there before but was part of the accident."

Penning, a former firefighter, said it was important to examine the aftermath of the crash when two fireballs caused devastation. "I was with senior fire officers and they have never seen this sort of accident happen before. We need to investigate that," said Penning.

Asked on BBC Radio 5 Live whether the incident would have any impact on government proposals to raise the motorway speed limit to 80mph, Penning said: "We will look very carefully at what comes out of this and see what sort of speeds were involved and put that into the consultation. It is wrong to jump on any bandwagon. We don't know what caused this accident and what happened after the accident started."